Mark Schreiner. 7 December 2013

Transcription

1 Simple Poverty Scorecard Poverty-Assessment Tool Tanzania Mark Schreiner 7 December 2013 This document is at SimplePovertyScorecard.com. Abstract The Simple Poverty Scorecard-brand poverty-assessment tool uses ten low-cost indicators from Tanzania s 2007 Household Budget Survey to estimate the likelihood that a household has expenditure below a given poverty line. Field workers can collect responses in about ten minutes. The scorecard s accuracy is reported for a range of poverty lines. The scorecard is a practical way for pro-poor programs in Tanzania to measure poverty rates, to track changes in poverty rates over time, and to segment clients for targeted services. Version note This paper corrects a problem with incomplete data for household members that is in the 8 March 2011 version. This affects the points for most indicators as well as all the poverty likelihoods and accuracy results. It also corrects errors in the 2005 PPP poverty lines that are in the 24 July 2012 version and in the 7 July 2013 version. These corrections affect all text and figures related to those poverty lines. Acknowledgements This paper was funded by the Ford Foundation via a grant to Grameen Foundation (GF). The data come from the National Bureau of Statistics. Thanks go to Marc Bavois, Sharlene Brown, Novati Buberwa, Lula Chen, Albina Chuwa, Clara Galeazzi, Lucas Kilyapeka, Ahmed Makbel, Rose Mungai, Sydney Neuschel, Trudy Owens, Mohamed Rajab, Jochen Schloesser, Dane Shikman, Jeff Toohig, Patrick Ward, and Cindy Yang. This scorecard was re-branded by GF as a Progress out of Poverty Index tool. The PPI is a performance-management tool that GF promotes to help organizations achieve their social objectives more effectively. Progress out of Poverty Index and PPI are Registered Trademarks of Innovations for Poverty Action. Simple Poverty Scorecard is a Registered Trademark of Microfinance Risk Management, L.L.C. for its brand of poverty-assessment tools.

2 Simple Poverty Scorecard Poverty-Assessment Tool Interview ID: Name Identifier Interview date: Participant: Country: TZA Field agent: Scorecard: 001 Service point: Sampling wgt.: Number of household members: Indicator Value Points Score 1. How many household members are A. Four or more 0 17-years-old or younger? B. Three 10 C. Two 15 D. One 20 E. None Do all children ages 6 to 17 attend A. No 0 school? B. Yes, or no children ages 6 to Can the female head/spouse read and write? A. No 0 B. Yes, but not in Kiswahili nor English 0 C. No female head/spouse 0 D. Yes, only in Kiswahili 6 E. Yes, in English (regardless of others) What is the main building material A. Earth 0 of the floor of the main dwelling? B. Concrete, cement, tiles, timber, or other What is the main building material of the roof of the main dwelling? 6. How many bicycles, mopeds, motorcycles, tractors, or motor vehicles does your household own? 7. Does your household own any radios or radio cassettes? 8. Does your household own any lanterns? 9. Does your household own any irons (charcoal or electric)? 10. How many tables does your household own? SimplePovertyScorecard.com A. Mud and grass 0 B. Grass, leaves, bamboo 8 C. Concrete, cement, metal sheets (GCI), asbestos sheets, tiles, or other 9 A. None 0 B. One 3 C. Two or more 11 A. No 0 B. Yes 6 A. No 0 B. Yes 6 A. No 0 B. Yes 5 A. None 0 B. One 2 C. Two 4 D. Three or more 6 Score:

3 Look-up table to convert scores to poverty likelihoods Poverty likelihood (%) by poverty line National USAID Intl PPP Score Food 100% 150% 200% 'Extreme' $1.25 $

4 Simple Poverty Scorecard Poverty-Assessment Tool Tanzania 1. Introduction Pro-poor programs in Tanzania can use the Simple Poverty Scorecard povertyassessment tool to estimate the likelihood that a household has expenditure below a given poverty line, to measure groups poverty rates at a point in time, to track changes in groups poverty rates over time, and to segment clients for targeted services. The direct approach to poverty measurement via surveys is difficult and costly. As a case in point, the 2007 Tanzania Household Budget Survey (HBS) asks more than 20 pages of questions over the course of two visits to a household, following that with visits every 2 3 days for a month to record the entries in a household diary of the purchase and consumption of about 400 possible income and expenditure items. In contrast, the indirect approach via the scorecard is simple, quick, and inexpensive. It uses 10 verifiable indicators (such as What is the main building material of the floor of the main dwelling? or Does your household own any irons (charcoal or electric)? ) to get a score that is highly correlated with poverty status as measured by the exhaustive survey. The scorecard differs from proxy means tests (Coady, Grosh, and Hoddinott, 2002) in that it is tailored to the capabilities and purposes not of national governments but rather of local, pro-poor organizations. The feasible poverty-measurement options 1

5 for these organizations are typically subjective and relative (such as participatory wealth ranking by skilled field workers) or blunt (such as rules based on land-ownership or housing quality). Results from these approaches are not comparable across organizations, they may be costly, and their accuracy is unknown. Pro-poor organizations can use the scorecard to measure the share of their participants who are below a given poverty line, such as the Millennium Development Goals $1.25/day line at 2005 purchase-power parity. USAID microenterprise partners can use it to report how many of their participants are among the poorest half of people below the national poverty line. Organizations can also use it to measure movement across a poverty line. In all these cases, the scorecard provides an expenditure-based, objective tool with known accuracy. While expenditure surveys are costly even for governments, many small, local organizations may be able to implement an inexpensive scorecard that can serve for monitoring and targeting. The statistical approach here aims to be understood by non-specialists. After all, if managers are to adopt the scorecard on their own and apply it to inform their decisions, they must first trust that it works. Transparency and simplicity build trust. Getting buy-in matters; proxy means tests and regressions on the determinants of poverty have been around for three decades, but they are rarely used to inform decisions, not because they do not work, but because they are presented (when they are presented at all) as tables of regression coefficients incomprehensible to lay people (with cryptic indicator names such as LGHHSZ_2, negative values, and many decimal places). 2

6 Thanks to the predictive-modeling phenomenon known as the flat maximum, simple scorecards are often about as accurate as complex ones. The technical approach here is innovative in how it associates scores with poverty likelihoods, in the extent of its accuracy tests, and in how it derives formulas for standard errors. Although these accuracy tests are simple and commonplace in statistical practice and in the for-profit field of credit-risk scoring, they have rarely been applied to poverty-assessment tools. The scorecard is based on the 2007 HBS conducted by Tanzania s National Bureau of Statistics (NBS) from 1 January 2007 to 31 December Indicators are selected to be: Inexpensive to collect, easy to answer quickly, and simple to verify Strongly correlated with poverty Liable to change over time as poverty status changes All points in the scorecard are non-negative integers, and total scores range from 0 (most likely below a poverty line) to 100 (least likely below a poverty line). Nonspecialists can collect data and tally scores on paper in the field in about ten minutes. The scorecard can be used to estimate three basic quantities. First, it can estimate a particular household s poverty likelihood, that is, the probability that the household has per-capita or per-adult-equivalent expenditure below a given poverty line. 1 The survey and thus the scorecard here applies only to mainland Tanzania. Data is available at 3

7 Second, the scorecard can estimate the poverty rate of a group of households at a point in time. This estimate is the average poverty likelihood among the households in the group. Third, the scorecard can estimate changes in the poverty rate for a group of households (or for two independent samples of households that are representative of the same population) between two points in time. This estimate is the change in the average poverty likelihood of the group(s) of households over time. The scorecard can also be used for targeting. To help managers choose an appropriate targeting cut-off for their purposes, this paper reports several measures of targeting accuracy for a range of possible cut-offs. This paper presents a single scorecard whose indicators and points are derived from household expenditure data and Tanzania s national ( basic needs ) poverty line. Scores from this one scorecard are calibrated to poverty likelihoods for seven poverty lines. The scorecard is constructed and calibrated using half of the data from the 2007 HBS, and its accuracy is validated on the other half. While all three scoring estimators are unbiased (that is, they match the true value on average in repeated samples when applied to the same population from which 4

8 the scorecard was built), they are like all predictive models biased to some extent when applied to a different population. 2 Thus, while the indirect scoring approach is less costly than the direct survey approach, it is also biased. (The survey approach is unbiased by assumption.) There is bias because scoring must assume that the future relationship between indicators and poverty will be the same as in the data used to build the scorecard. Of course, this assumption ubiquitous and inevitable in predictive modeling holds only partly. When applied to the validation sample with bootstrap samples of n = 16,384, the difference between scorecard estimates of groups poverty rates and the true rates at a point in time is 1.3 percentage points for the national line, and the average absolute difference across all seven lines is 0.7 percentage points. These differences are due to sampling variation and not bias; the average of each difference would be zero if the whole 2007 HBS were to be repeatedly redrawn and divided into sub-samples before repeating the entire process of building and calibrating scorecards. 3 The 90-percent confidence intervals for these estimates are +/ 0.7 percentage points or less. For n = 1,024, the 90-percent intervals are +/ 2.5 percentage points or less. 2 Important examples include nationally representative samples at a different point in time or non-nationally representative sub-groups (Tarozzi and Deaton, 2009). 3 Most differences for Tanzania seems to be due to constructing the scorecard with an adult-equivalent poverty line and then applying it to per-capita poverty lines. 5

9 Section 2 below describes data and poverty lines. Sections 3 and 4 describe scorecard construction and offer guidelines for use in the field. Sections 5 and 6 detail the estimation of households poverty likelihoods and of groups poverty rates at a point in time. Section 7 discusses estimating changes in poverty rates over time, and Section 8 covers targeting. Section 9 places the new scorecard here in the context of existing exercises for Tanzania, and Section 10 is a summary. 6

10 2. Data and poverty lines This section discusses the data used to construct and validate the scorecard. It also presents the poverty lines to which scores are calibrated. 2.1 Data The scorecard is based on data from the 2007 HBS. Households are randomly divided into two sub-samples (Figure 2): Construction and calibration for selecting indicators and points and for associating scores with poverty likelihoods Validation for testing accuracy on data not used in construction or calibration 2.2 Poverty rates and poverty lines Rates As a general definition, the poverty rate is the share of people in a group who live in households whose total household expenditure (divided by the number of adult equivalents or by the number of household members) is below a given poverty line. Beyond this general definition, there two special cases, household-level poverty rates and person-level poverty rates. With household-level rates, each household is counted as if it had only one person, regardless of true household size, so all households are counted equally. With person-level rates (the head-count index ), each household is weighted by the number of people in it, so larger households count more. 7

11 For example, consider a group of two households, the first with one member and the second with two members. Suppose further that the first household has per-capita expenditure (or per-adult-equivalent expenditure) above a poverty line (it is non-poor ) and that the second household has per-capita expenditure (or per-adult-equivalent expenditure) below a poverty line (it is poor ). The household-level rate counts both households as if they had only one person and so gives a poverty rate of 1 (1 + 1) = 50 percent. In contrast, the person-level rate weighs each household by the number of people in it and so gives a poverty rate of 2 (1 + 2) = 67 percent. Whether the household-level rate or the person-level rate is more relevant depends on the situation. If an organization s participants include all the people in a household, then the person-level rate is relevant. Governments, for example, are concerned with the well-being of people, regardless of how those people are arranged in households, so governments typically report person-level poverty rates. If an organization has only one participant per household, however, then the household-level rate may be relevant. For example, if a microlender has only one borrower in a household, then it might prefer to report household-level poverty rates. Figure 2 reports poverty lines and poverty rates at both the household-level and the person-level for the three regions of Dar es Salaam, Other Urban, and Rural, as well as for mainland Tanzania. The scorecard is constructed using the 2007 HBS and household-level lines, scores are calibrated to household-level poverty likelihoods, and accuracy is measured for household-level rates. 8

12 Person-level poverty rates can be estimated by taking a household-size-weighted average of the household-level poverty likelihoods. It is possible to construct, calibrate, and validate a scorecard based on person-level weights, but it is not done here Poverty lines Tanzania s food poverty line is TZS359 per adult equivalent per day. 4 This is defined as the cost of 2,200 calories using food items consumed by the poorest half of the population in the 2000/1 HBS. This is adjusted to 2007 by a factor of 1.93 to account for price changes, and it is also adjusted for price differences across the three regions in 2007 (NBS, 2009 and 2002b). Tanzania s national ( basic needs ) poverty line of TZS492 per adult equivalent per day is defined as the food line, divided by the share of expenditure among the poorest quartile of households that goes to food (NBS, 2002b). This was 73 percent in the 2000/1 HBS, giving a national line of TZS359/0.73 = TZS492. This adjustment accounts for the fact that households below the 2,200-calorie benchmark nevertheless have some expenditure on (apparently necessary) non-food items (Ravaillon, 1998). The scorecard here is constructed using the national poverty line (sometimes called 100% of the national line ). For mainland Tanzania, the national line implies a household-level poverty rate of 26.6 percent and a person-level poverty rate of The adult-equivalence scale appears in NBS (2002a). 9

13 percent. For the food poverty line, the household-level poverty rate for mainland Tanzania is 12.6 percent, and the person-level rate is 16.6 percent. Because local pro-poor organizations may want to use different or various poverty lines, this paper calibrates scores from its single scorecard to poverty likelihoods for seven lines: Food National 150% of national 200% of national USAID extreme $1.25/day 2005 PPP $2.50/day 2005 PPP The USAID extreme line (U.S. Congress, 2002) is defined as the median percapita expenditure of people (not households) below the national line in a given poverty-line region. The $1.25/day 2005 PPP line is derived from: 2005 PPP exchange rate for individual consumption expenditure by households of TZS per $1 (World Bank, 2008) Average mainland Tanzania Consumer Price Index (CPI) for 2005 of Average CPI for 2007 of category&id=1 38:summary-cpi&download=411:cpi-summary releases&itemid=106, retrieved 28 January

14 The $1.25/day 2005 PPP line for mainland Tanzania applied to the 2007 HBS is (Sillers, 2006): CPI 2005 PPP exchange rate $1.25 CPI TZS $ TZS. $ Ave Ave The $2.50/day 2005 PPP line is twice the $1.25/day 2005 PPP line. The mainland-tanzania $1.25/day line of TZS692 is adjusted for cost-of-living difference across Tanzania s three poverty-line regions. A given poverty-line region s $1.25/day line is the mainland-tanzania line of TZS692, divided by the value of the national poverty line applicable in that region, and then divided by the average personweighted national line across all regions. 11

15 3. Scorecard construction About 75 potential indicators are initially prepared in the areas of: Family composition (such as household size) Education (such as literacy of the female head/spouse) Housing (such as the type of floor or roof) Ownership of durable goods (such as lanterns or irons) Figure 3 lists all potential indicators, ranked by the entropy-based uncertainty coefficient that measures how well a given indicator predicts poverty on its own (Goodman and Kruskal, 1979). The scorecard also aims to measure changes in poverty through time. This means that, when selecting indicators and holding other considerations constant, preference is given to more sensitive indicators. For example, ownership of a lantern or an iron is probably more likely to change in response to changes in poverty than is the marital status of the male head/spouse. The scorecard itself is built using the national poverty line and Logit regression on the construction sub-sample. Indicator selection uses both judgment and statistics. The first step is to use Logit to build one scorecard for each candidate indicator. Each scorecard s accuracy is taken as c, a measure of ability to rank by poverty status (SAS Institute Inc., 2004). One of these one-indicator scorecards is then selected based on several factors (Schreiner et al., 2004; Zeller, 2004), including improvement in accuracy, likelihood of acceptance by users (determined by simplicity, cost of collection, and face validity in 12

16 terms of experience, theory, and common sense), sensitivity to changes in poverty status, variety among indicators, and verifiability. A series of two-indicator scorecards are then built, each based on the oneindicator scorecard selected from the first step, with a second candidate indicator added. The best two-indicator scorecard is then selected, again based on c and judgment. These steps are repeated until the scorecard has 10 indicators. The final step is to transform the Logit coefficients into non-negative integers such that total scores range from 0 (most likely below a poverty line) to 100 (least likely below a poverty line). This algorithm is the Logit analogue to the common R 2 -based stepwise leastsquares regression. It differs from naïve stepwise in that the criteria for selecting indicators include not only statistical accuracy but also judgment and non-statistical factors. The use of non-statistical criteria can improve robustness through time and helps ensure that indicators are simple and make sense to users. The single scorecard here applies to all of mainland Tanzania. Evidence from India and Mexico (Schreiner, 2006 and 2005a), Sri Lanka (Narayan and Yoshida, 2005), and Jamaica (Grosh and Baker, 1995) suggests that segmenting scorecards by urban/rural does not improve targeting accuracy much, although it may improve the accuracy of estimates of poverty rates (Tarozzi and Deaton, 2009). 13

17 4. Practical guidelines for scorecard use The main challenge of scorecard design is not to maximize statistical accuracy but rather to improve the chances that scoring is actually used in practice (Schreiner, 2005b). When scoring projects fail, the reason is not usually statistical inaccuracy but rather the failure of an organization to decide to do what is needed to integrate scoring in its processes and to learn to use it properly (Schreiner, 2002). After all, most reasonable scorecards have similar targeting accuracy, thanks to the empirical phenomenon of the flat maximum (Falkenstein, 2008; Hand, 2006; Baesens et al., 2003; Lovie and Lovie, 1986; Kolesar and Showers, 1985; Stillwell, Barron, and Edwards, 1983; Dawes, 1979; Wainer, 1976; Myers and Forgy, 1963). The bottleneck is less technical and more human, not statistics but organizational-change management. Accuracy is easier to achieve than adoption. The scorecard here is designed to encourage understanding and trust so that users will adopt it and use it properly. Of course, accuracy matters, but it is balanced against simplicity, ease-of-use, and face validity. Programs are more likely to collect data, compute scores, and pay attention to the results if, in their view, scoring does not make a lot of extra work and if the whole process generally seems to make sense. 14

18 To this end, the scorecard here fits on one page. The construction process, indicators, and points are simple and transparent. Extra work is minimized; nonspecialists can compute scores by hand in the field because the scorecard has: Only 10 indicators Only categorical indicators Simple weights (non-negative integers, and no arithmetic beyond addition) A field worker using the paper scorecard would: Record participant identifiers and household size Read each question from the scorecard Circle the response and its point value Write the point value in the far-right column Add up the points to get the total score Implement targeting policy (if any) Deliver the paper scorecard to a central office for filing or data entry and analysis Of course, field workers must be trained. The quality of outputs depends on the quality of inputs. If organizations or field workers gather their own data and believe that they have an incentive to exaggerate poverty rates (for example, if funders reward them for higher poverty rates), then it is wise to do on-going quality control via data review and audits (Matul and Kline, 2003). 6 IRIS Center (2007) and Toohig (2008) are useful nuts-and-bolts guides for budgeting, training field workers and supervisors, logistics, sampling, interviewing, piloting, recording data, and controlling quality. In particular, while collecting scorecard indicators is relatively easier than alternatives, it is still absolutely difficult. Training and explicit definitions of terms and 6 If an organization does not want field workers to know the points associated with indicators, then they can use the version of the scorecard without points and apply the points later at the central office. 15

19 concepts in the scorecard is essential. For the example of Nigeria, Onwujekwe, Hanson, and Fox-Rushby (2006) found distressingly low inter-rater and test-retest correlations for indicators as seemingly simple and obvious as whether the household owns an automobile. At the same time, Grosh and Baker (1995) find that gross underreporting of assets does not affect targeting. For the first stage of targeting in a conditional cashtransfer program in Mexico, Martinelli and Parker (2007) find that underreporting [of asset ownership] is widespread but not overwhelming, except for a few goods... [and] overreporting is common for a few goods, which implies that self-reporting may lead to the exclusion of deserving households (pp ). Still, as is done in Mexico in the second stage of its targeting process, most false self-reports can be corrected by field agents who verify responses with a home visit, and this is the suggested procedure for the scorecard in Tanzania. In terms of sampling design, an organization must make choices about: Who will do the scoring How scores will be recorded What participants will be scored How many participants will be scored How frequently participants will be scored Whether scoring will be applied at more than one point in time Whether the same participants will be scored at more than one point in time the exercise. In general, the sampling design should follow from the organization s goals for The non-specialists who apply the scorecard with participants in the field can be: Employees of the organization Third-party contractors 16

20 Responses, scores, and poverty likelihoods can be recorded: On paper in the field and then filed at an office On paper in the field and then keyed into a database or spreadsheet at an office On portable electronic devices in the field and downloaded to a database Given a population of interest, the subjects to be scored can be: All participants A representative sample of all participants All participants in a representative sample of branches A representative sample of all participants in a representative sample of branches If not determined by other factors, the number of participants to be scored can be derived from sample-size formulas (presented later) for a desired level of confidence and a desired confidence interval. Frequency of application can be: At in-take of new clients only (precluding measuring change in poverty rates) As a once-off project for current participants (precluding measuring change) Once a year or at some other fixed time interval (allowing measuring change) Each time a field worker visits a participant at home (allowing measuring change) When the scorecard is applied more than once in order to measure change in poverty rates, it can be applied: With a different set of participants With the same set of participants An example set of choices are illustrated by BRAC and ASA, two microlenders in Bangladesh who each have more than 7 million participants and who are applying the Simple Poverty Scorecard tool for Bangladesh (Chen and Schreiner, 2009b). Their design is that loan officers in a random sample of branches score all participants each 17

21 time they visit a homestead (about once a year) as part of their standard due diligence prior to loan disbursement. Responses are recorded on paper in the field before being sent to a central office to be entered into a database. ASA s and BRAC s sampling plans cover 50, ,000 participants each. 18

22 5. Estimates of household poverty likelihoods The sum of scorecard points for a household is called the score. For Tanzania, scores range from 0 (most likely below a poverty line) to 100 (least likely below a poverty line). While higher scores indicate less likelihood of being below a poverty line, the scores themselves have only relative units. For example, doubling the score does not double the likelihood of being above a poverty line. To get absolute units, scores must be converted to poverty likelihoods, that is, probabilities of being below a poverty line. This is done via simple look-up tables. For the example of the national line, scores of have a poverty likelihood of 29.6 percent, and scores of have a poverty likelihood of 22.8 percent (Figure 4). The poverty likelihood associated with a score varies by poverty line. For example, scores of are associated with a poverty likelihood of 29.6 percent for the national line but 13.2 percent for the food line Calibrating scores with poverty likelihoods A given score is associated ( calibrated ) with a poverty likelihood by defining the poverty likelihood as the share of households in the calibration sub-sample who have the score and who are below a given poverty line. 7 Starting with Figure 4, most figures have seven versions, one for each poverty line. To keep them straight, they are grouped by poverty line. Single tables that pertain to all poverty lines are placed with the tables for the national line. 19

23 For the example of the national line (Figure 5), there are 8,395 households in the calibration sub-sample with a score of 35 39, of whom 2,485 are below the poverty line. The estimated poverty likelihood associated with a score of is then 29.6 percent, because 2,485 8,395 = 29.6 percent. To illustrate with the national line and a score of 40 44, there are 10,897 households in the calibration sample, of whom 2,482 are below the line (Figure 5). Thus, the poverty likelihood for this score is 2,482 10,897 = 22.8 percent. The same method is used to calibrate scores with estimated poverty likelihoods for the other poverty lines. Figure 6 shows, for all scores, the likelihood that expenditure falls in a range demarcated by two adjacent poverty lines. There are two versions of Figure 6, one for the national poverty lines in units of adult equivalents per day, and another for the USAID extreme line and the 2005 PPP lines in units of people per day. For the per-adult-equivalent national lines, the daily expenditure of someone with a score of falls in the following ranges with probability: 13.2 percent below the food line 16.4 percent between the food line and 100% of the national line 28.3 percent between 100% and 150% of the national line 23.1 percent between 150% and 200% of the national line 19.0 percent above 200% of the national line 20

24 For the example of the per-capita lines, the daily expenditure of someone with a score of falls in the following ranges with probability: 12.9 percent below the USAID extreme line 61.0 percent between the USAID extreme line and $1.25/day 2005 PPP line 24.0 percent between the $1.25/day and $2.50/day 2005 PPP lines 2.1 percent above the $2.50/day 2005 PPP line Even though the scorecard is constructed partly based on judgment, the calibration process produces poverty likelihoods that are objective, that is, derived from survey data on expenditure and quantitative poverty lines. The poverty likelihoods would be objective even if indicators and/or points were selected without any data at all. In fact, objective scorecards of proven accuracy are often based only on judgment (Fuller, 2006; Caire, 2004; Schreiner et al., 2004). Of course, the scorecard here is constructed with both data and judgment. The fact that this paper acknowledges that some choices in scorecard construction as in any statistical analysis are informed by judgment in no way impugns the objectivity of the poverty likelihoods, as this depends on using data in score calibration, not on using data (and nothing else) in scorecard construction. Although the points in Tanzania s scorecard are transformed coefficients from a Logit regression, scores are not converted to poverty likelihoods via the Logit formula of score x ( score ) 1. This is because the Logit formula is esoteric and difficult to compute by hand. Non-specialists find it more intuitive to define the poverty likelihood as the share of households with a given score in the calibration sample who are below a poverty line. In the field, converting scores to poverty likelihoods requires 21

25 no arithmetic at all, just a look-up table. This non-parametric calibration can also improve accuracy, especially with large calibration samples. 5.2 Accuracy of estimates of poverty likelihoods As long as the relationship between indicators and poverty does not change, this calibration process produces unbiased estimates of poverty likelihoods. Unbiased means that in repeated samples from the same population, the average estimate matches the true poverty likelihood. The scorecard also produces unbiased estimates of poverty rates at a point in time and of changes in poverty rates between two points in time. 8 Of course, the relationship between indicators and poverty changes with time, so the scorecard applied after 2007 (as it must be in practice) will generally be biased. 8 This follows because these estimates of groups poverty rates are linear functions of the unbiased estimates of households poverty likelihoods. 22

26 How accurate are estimates of poverty likelihoods given the assumption of a sample that is representative of mainland Tanzania in 2007? To measure, the scorecard is applied to 1,000 bootstrap samples of size n = 16,384 from the validation sub-sample. Bootstrapping entails (Efron and Tibshirani, 1993): Score each household in the validation sample Draw a new bootstrap sample with replacement from the validation sample For each score, compute the true poverty likelihood in the bootstrap sample, that is, the share of households with the score and expenditure below a poverty line For each score, record the difference between the estimated poverty likelihood (Figure 4) and the true poverty likelihood in the bootstrap sample Repeat the previous three steps 1,000 times For each score, report the average difference between estimated and true poverty likelihoods across the 1,000 bootstrap samples For each score, report the two-sided interval containing the central 900, 950, or 990 differences between estimated and true poverty likelihoods For each score range, Figure 7 shows the average difference between estimated and true poverty likelihoods as well as confidence intervals for the differences. For the national line, the average poverty likelihood across bootstrap samples for scores of in the validation sample is too high by 0.9 percentage points (Figure 7). For scores of 40 44, the estimate is too low by 2.9 percentage points. 9 For the validation sample, the 90-percent confidence interval for the differences for scores of is +/ 2.2 percentage points (Figure 7). This means that in 900 of 1,000 bootstraps, the difference between the estimate and the true value is between These differences are not zero, despite the estimator s unbiasedness, because the bootstrap estimates come from a single sample (the 2007 HBS). Their average difference would be zero if samples were repeatedly drawn from the population and split into subsamples before repeating the entire scorecard-building process. 23

27 and +3.1 percentage points (because = 1.3, and = +3.1). In 950 of 1,000 bootstraps (95 percent), the difference is / 2.7 percentage points, and in 990 of 1,000 bootstraps (99 percent), the difference is / 3.3 percentage points. For many score ranges, Figure 7 shows differences (some of them large) between estimated poverty likelihoods and true values. This is because the validation sub-sample is a single sample that thanks to sampling variation differs in distribution from the construction/calibration sub-samples and from mainland Tanzania s population. For targeting, however, what matters is less the difference in all score ranges and more the difference in score ranges just above and below the targeting cut-off. This mitigates the effects of bias and sampling variation on targeting (Friedman, 1997). Section 8 below looks at targeting accuracy in detail. Of course, if estimates of groups poverty rates are to be usefully accurate, then errors for individual households must largely cancel out. As discussed later, this is generally the case. By construction, the scorecard here is unbiased. It may still, however, be overfit when applied after the end of the HBS fieldwork in December That is, it may fit the HBS data so closely that it captures not only some timeless patterns but also some random patterns that, due to sampling variation, show up only in the HBS. Or the scorecard may be overfit in the sense that its bias is highly sensitive to changes over time in the relationship between indicators and poverty. 24

28 Overfitting can be mitigated by simplifying the scorecard and by not relying only on data but rather also considering experience, judgment, and theory. Of course, the scorecard here does this. Bootstrapping can also mitigate overfitting by reducing (but not eliminating) dependence on a single sampling instance. Combining scorecards can also help, at the cost of greater complexity. Most errors in individual households likelihoods, however, cancel out in the estimates of groups poverty rates (see later sections). Furthermore, much of the differences may come from non-scorecard sources such as changes in the relationship between indicators and poverty, sampling variation, and inconsistencies in data quality. These factors can be addressed only by improving data quantity and quality (which is beyond the scope of the scorecard) or by reducing overfitting (which likely has limited returns, given the scorecard s parsimony). 25

29 6. Estimates of a group s poverty rate at a point in time A group s estimated poverty rate at a point in time is the average of the estimated poverty likelihoods of the individual households in the group. To illustrate, suppose a program samples three households on Jan. 1, 2011 and that they have scores of 20, 30, and 40, corresponding to poverty likelihoods of 53.5, 38.7, and 22.8 percent (national line, Figure 4). The group s estimated poverty rate is the households average poverty likelihood of ( ) 3 = 38.3 percent Accuracy of estimated poverty rates at a point in time For the Tanzania scorecard applied to the validation sample with n = 16,384, the absolute differences between the estimated poverty rate at a point in time and the true rate for the seven poverty lines are 1.3 percentage points or less (Figure 8, summarizing Figure 9 across poverty lines). The average absolute difference across the seven poverty lines is 0.7 percentage points. At least part of these differences is due to sampling variation in the validation sample and in the division of the 2007 HBS into two sub-samples. In terms of precision, the 90-percent confidence interval for a group s estimated poverty rate at a point in time with n = 16,384 is +/ 0.7 percentage points or less 10 The group s poverty rate is not the poverty likelihood associated with the average score. Here, the poverty likelihood associated with the average score of 30 is 46.1 percent, which differs from the average of the three poverty likelihoods associated with each of the three scores (42.9 percent). 26

30 (Figure 8). This means that in 900 of 1,000 bootstraps of this size, the difference between the estimate and the true value is within 0.7 percentage points of the average difference. In the specific case of 100% of the national line and the validation sample, 90 percent of all samples of n = 16,384 produce estimates that differ from the true value in the range of = 1.9 to = 0.7 percentage points. This is because 1.3 is the average difference, and +/ 0.6 is its 90-percent confidence interval. The average difference is 1.3 because the average scorecard estimate is too low by 1.3 percentage points; the scorecard tends to estimate a poverty rate of 25.2 percent for the validation sample, but the true value is 26.5 percent (Figure 2). 6.2 Formula for standard errors for estimates of poverty rates How precise are the point-in-time estimates? Because they are averages of binary (0/1, or poor/non-poor) variables, the estimates (in large samples) have a Normal distribution and can be characterized by their average difference vis-à-vis true values together with the standard error of the average difference. 27

31 To derive a formula for the standard errors of estimated poverty rates at a point in time from indirect measurement via scorecards (Schreiner, 2008a), note that the textbook formula (Cochran, 1977) that relates confidence intervals with standard errors in the case of direct measurement of rates is c / z, where: c is a confidence interval as a proportion (e.g., 0.02 for +/ 2 percentage points), z is from the Normal distribution and is 1.64 for confidence levels of 90 percent 1.96 for confidence levels of 95 percent, 2.58 for confidence levels of 99 percent σ is the standard error of the estimated poverty rate, that is, p ( 1 p), n p is the proportion of households below the poverty line in the sample, and n is the sample size. For example, this implies that for a sample n of 16,384 with 90-percent confidence (z = 1.64) and a poverty rate p of 26.7 percent (the average poverty rate in the construction and calibration samples in Figure 2 for the national line), the confidence interval c is p (1 p) ( ) / z / / n 16,384 percentage points. The scorecard, however, does not measure poverty directly, so this formula is not immediately applicable. To derive a formula for the Tanzania scorecard, consider Figure 9, which reports empirical confidence intervals c for the differences for the scorecard applied to 1,000 bootstrap samples of various sample sizes from the validation 28

32 sample. For n = 16,384 and the national line, the 90-percent confidence interval is percentage points. 11 Thus, the 90-percent confidence interval with n = 16,384 is percentage points for the Tanzania scorecard and percentage points for direct measurement. The ratio of the two intervals is = Now consider the same case, but with n = 8,192. The confidence interval under direct measurement is ( ) / / percentage points. The 8,192 empirical confidence interval with the Tanzania scorecard (Figure 9) is percentage points. Thus for n = 8,192, the ratio of the two intervals is = This ratio is 1.15 for both n = 8,182 and 1.13 for n = 16,384. Across all sample sizes of 256 or more in Figure 9, the average ratio turns out to be 1.12, implying that confidence intervals for indirect estimates of poverty rates via the Tanzania scorecard and this poverty line are about 12 percent wider than confidence intervals for direct estimates via the 2007 HBS. This 1.12 appears in Figure 8 as the α factor because if α = 1.12, then the formula relating confidence intervals c and standard errors σ for the Tanzania scorecard is c / z. In turn, the formula for the standard error σ for point-in-time estimates of poverty rates via scoring is p ( 1 p). n 11 Due to rounding, Figure 9 displays 0.6, not

33 In general, α can be more or less than When α is less than 1.00, it means that the scorecard is more precise than direct measurement. This is the case for four of the seven poverty lines in Figure 8. The formula relating confidence intervals with standard errors for the scorecard can be rearranged to give a formula for determining sample size before measurement. 12 If pˆ is the expected poverty rate before measurement, then the formula for sample size n based on the desired confidence level that corresponds to z and the desired confidence z. c interval +/ c is n pˆ 1 pˆ 2 To illustrate how to use this, suppose c = and z = 1.64 (90-percent confidence). Then the formula gives n ( ) = 273, not too far from the sample size of 256 observed for these parameters in Figure 9 for the national line. Of course, the α factors in Figure 8 are specific to Tanzania, its poverty lines, its poverty rates, and this scorecard. The approach to deriving the formulas, however, is valid for any scorecard following the approach in this paper Although USAID has not specified required confidence levels or intervals, IRIS Center (2007a and 2007b) says that a sample size of n = 300 is sufficient for USAID reporting. Given α = 0.95 for the $1.25/day 2005 PPP line to be used for reporting by USAID microenterprise partners in Tanzania, an expected before-measurement poverty rate of 61.3 percent (the all-tanzania $1.25/day rate for 2007), and a confidence level of 90 percent, then n = 300 implies a confidence interval of = ±4.4 percentage points ( )

34 In practice after the end of fieldwork for the HBS in December 2007, an organization would select a poverty line (say, the national line), select a desired confidence level (say, 90 percent, or z = 1.64), select a desired confidence interval (say, +/ 2.0 percentage points, or c = 0.02), make an assumption about pˆ (perhaps based on a previous measurement such as the 26.6 percent national average in the 2007 HBS in Figure 2), look up α (here, 1.12), assume that the scorecard will still work in the future and/or for non-nationally representative sub-groups, 13 and then compute the required sample size. In this illustration, n = 1, This paper reports accuracy for the scorecard applied to the validation sample, but it cannot test accuracy for later years or for other groups. Performance after 2007 will resemble that in the 2007 HBS with deterioration to the extent that the relationships between indicators and poverty status change over time. 31

35 7. Estimates of changes in group poverty rates over time The change in a group s poverty rate between two points in time is estimated as the change in the average poverty likelihood of the households in the group. With data only from the 2007 HBS, this paper cannot test estimates of change over time for Tanzania, and it can only suggest approximate formulas for standard errors. Nevertheless, the relevant concepts are presented here because, in practice, pro-poor organizations can use the scorecard to get data to measure change through time. 7.1 Warning: Change is not impact Scoring can estimate change. Of course, poverty could get better or worse, and scoring does not indicate what caused change. This point is often forgotten or confused, so it bears repeating: the scorecard simply estimates change, and it does not, in and of itself, indicate the reason for the change. In particular, estimating the impact of program participation requires knowing what would have happened to participants if they had not been participants. Knowing this requires either strong assumptions or a control group that resembles participants in all ways except participation. To belabor the point, the scorecard can help estimate program impact only if there is some way to know what would have happened in the absence of the program. And that information must come from somewhere beyond the scorecard. 32

36 7.2 Calculating estimated changes in poverty rates over time Consider the illustration begun in the previous section. On Jan. 1, 2011, a program samples three households who score 20, 30, and 40 and so have poverty likelihoods of 53.5, 38.7, and 22.8 percent (national line, Figure 4). The group s baseline estimated poverty rate is the households average poverty likelihood of ( ) 3 = 38.3 percent. After baseline, two sampling approaches are possible for the follow-up round: Score a new, independent sample, measuring change by cohort across samples Score the same sample at follow-up as at baseline By way of illustration, suppose that a year later on Jan. 1, 2012, the program samples three additional households who are in the same cohort as the three households originally sampled (or suppose that the program scores the same three original households a second time) and finds that their scores are 25, 35, and 45 (poverty likelihoods of 48.4, 29.6, and 21.2 percent, national line, Figure 4). Their average poverty likelihood at follow-up is now ( ) 3 = 33.1 percent, an improvement of = 5.2 percentage points. 14 This suggests that about one in twenty participants in this hypothetical example crossed the poverty line in Among those who started below the line, about one in seven ( = 13.6 percent) on net ended up above the line Of course, such a huge reduction in poverty in one year is unlikely, but this is just an example to show how the scorecard can be used to estimate change. 15 This is a net figure; some people start above the line and end below it, and vice versa. 33

37 7.3 Accuracy for estimated change in two independent samples With only the 2007 HBS, it is not possible to measure the accuracy of scorecard estimates of changes in groups poverty rates over time. In practice, of course, local propoor organizations can still apply the Tanzania scorecard to estimate change. The rest of this section suggests approximate formulas for standard errors and sample sizes that may be used until there is additional data. For two equal-sized independent samples, the same logic as above can be used to derive a formula relating the confidence interval c with the standard error σ of a scorecard s estimate of the change in poverty rates over time: c 2 p (1 p) / z / z. n z, c, and p are defined as above, n is the sample size at both baseline and followup, 17 and α is the average (across a range of bootstrapped sample sizes) of the ratio of the observed confidence interval from a scorecard and the theoretical confidence interval under direct measurement. 16 The scorecard does not reveal the reasons for this change. 17 This means that, for a given precision and with direct measurement, estimating the change in a poverty rate between two points in time requires four times as many measurements (not twice as many) as does estimating a poverty rate at a point in time. 34